The present invention relates to an antenna formed by integrating a plurality of radiating elements on a board and for transmitting/receiving a plurality of frequencies.
Recently, mobile-satellite communications such as between mobile bodies including airplanes, vessels, or cars and satellites have been prevailing. Highly-efficient antennas mounted to the mobile bodies are developed by studying a shape of radiator elements of the antennas to be small in size. This study produces a meander-line antenna or a plate-type inverse F antenna. For instance, Japanese Patent Application Non-examined Publication No. H06-90108 discloses one of the products.
FIG. 15 illustrates a conventional antenna, and FIG. 16 illustrates that the antenna is mounted to a mobile body. FIG. 17 shows a relation between return losses and frequencies. In FIG. 15, antenna 200 comprises the following elements:
(a) Insulation board 200;
(b) Radiator element 201 and formed by meander-line of length L and width W disposed on insulation board 200;
(c) Feed line 202; and
(d) Feeding point 203 disposed near the center of element 201.
The frequency characteristic of antenna 200, as shown in FIG. 17, has a single resonance frequency f0, and frequency f0 is determined mainly by element""s length L. The frequency relative bandwidth of antenna 200 is about several percent.
FIG. 18 shows another conventional antenna, which includes two independent antenna sections 210 and 220 having different resonance frequencies. Antenna section 210 comprises radiator element 211, short-circuiting plate 212, feeding section 213, and grounding plate 214. Radiator 211 shapes in a long and narrow rectangular plate, and short-circuiting plate 212 couples electrically radiator element 211 with grounding plate 214. Feeding section 213 is disposed at a side end of element 211 as shown in FIG. 18. Antenna section 210 has a resonance frequency f1 determined by the sum (L1+H1) of length L1 of element 211 and the height H1 of short-circuiting plate 212. Antenna section 220 is structured in the same way and has resonance frequency f2 determined by the sum (L2+H2) of length L2 of element 221 and the height H2 of short-circuiting plate 222. The frequency relative bandwidth of this antenna is also several percent, and if the radiator element is shortened for downsizing the antenna, the relative bandwidth further narrows.
In general, as discussed above, the conventional antenna has a narrow frequency relative bandwidth. Therefore, when a transmitting band and a receiving band are greater than the relative bandwidth, an antenna including two sections, one for transmitting and the other for receiving, is required. For example, in the mobile satellite communication system (ORBCOMM system) assigned by World Administrative Radio Conference 1992, where several dozens of low-earth-orbital satellites perform data communication between the ground and the satellites, frequencies for uplink/downlink (137.0-138.0 MHz/148.0-150.05 MHz) are used. Also, a conventional antenna cannot cover these two frequency-bands, therefore, an antenna including two sections, one for transmitting and the other for receiving is required.
When antennas independently dedicated to transmitting and receiving are used, these antennas should be separately mounted to a mobile body, such as a car or a container. Accordingly, two feeding points and two feed lines are required, and wiring job should be doubled, which makes a mounting job very complicated. The conventional antenna is ca. 0.5 m tall, when this is mounted to a container, it is hard to put the antenna into a clearance between the containers. Further, when the antenna is mounted vertically to a car, the antenna is vulnerable to damaged caused by wind pressure or interference with other members. When the antenna is mounted horizontally to a car, the metal sheet of the car influences the antenna so that an antenna impedance changes, its resonance frequency shifts, or impedance matching between the antenna and its feeding line is disordered, thereby the antenna does not work properly.
An antenna being capable of transmitting and receiving a plurality of frequencies is provided. This antenna is small, thin, and easy-to-mount. The antenna is also characterized by high production-efficiency.
This antenna comprises a ground plate and a plurality of radiator elements shaping with a rectangular shape, where a first end of the antenna is short-circuited to the grounding plate, the elements which are disposed separately from the ground plate. The longitudinal lengths of the elements are set within the range of xcex/8-3xcex/8 so that a working frequency of respective elements meet given frequencies. One single antenna thus can transmit and receive frequencies within a desired frequency bandwidth. Further, couplers between a plurality of elements are provided so that only one radiator element can be fed power, and the other elements are fed indirectly. This structure allows a number of feeding points and feed lines to be reduced.